I've tried SEO Ultimate and P3 on a test site. There doesn't appear to be a conflict with the plugins at a code level. To get rid of the WSOD, please remove the plugins/p3-profiler folder from your site, and mu-plugins/p3-profiler.php file, too, if it's there.

To troubleshoot what's going on, you'll need to look in your server logs, or turn on debugging in wp-config.php.

My initial guess is that you should consider raising your php memory limit. Each plugin you activate raises your site's memory needs and you'll eventually turn activate one that pushes you over the limit and you'll get an out of memory error.